1. Field of the Invention
The present invention relates to a solid electrolytic capacitor. Particularly, it relates to a solid electrolytic capacitor wherein a conductive polymer compound is used as the solid electrolyte.
2. Discussion of Background
In recent years, electronic equipments and devices tend to be small in size and light in weight. Accordingly, small size capacitors having a large capacity and a low impedance at a high frequency region have been desired. As capacitors for such high frequency use, mica capacitors, film capacitors or ceramic capacitors have been used. However, none of these capacitors are suitable for a large capacity.
On the other hand, as small size capacitors having a large capacity, there are aluminum electrolytic capacitors or tantalum capacitors.
The aluminum electrolytic capacitors have a merit that products having a large capacity can be obtained at low costs. However, since they employ an electrolyte, they have drawbacks that the capacity deteriorates due to the evaporation of the electrolyte as time passes, and they are inferior in the high frequency characteristics.
On the other hand, tantalum solid electrolytic capacitors overcome such drawbacks of the aluminum electrolytic capacitors such as the deterioration of the capacity, by using e.g. solid manganese dioxide as the electrolyte. However, this solid electrolyte is prepared by impregnating and depositing an aqueous manganese nitrate solution to a valve metal, followed by thermal decomposition of manganese nitrate at a temperature of about 350.degree. C. In order to increase the amount of the deposition of manganese dioxide, it is necessary to repeat the steps of impregnation and thermal decomposition from a few times to a few tens times. Consequently, there have been drawbacks such that the dielectric oxide layer is likely to be damaged during the thermal decomposition, and the mending capability of the manganese dioxide layer is low.
In order to overcome such drawbacks, it has been proposed e.g. in Japanese Unexamined Patent Publication No. 17609/1983 to employ a 7,7,8,8-tetracyanoquinodimethane complex salt (hereinafter referred to simply as the TCNQ complex salt) as an organic solid electrolyte having good conductivity and excellent properties for mending the dielectric oxide layer. This Japanese Unexamined Patent Publication No. 17609/1983 discloses a capacitor wherein an organic semiconductor comprising the TCNQ complex salt between TCNQ and an isoquinolinium cation having the N-position substituted by an alkyl group, is used as the solid electrolyte, and the solid electrolyte layer is formed by dipping a capacitor element in the molten TCNQ salt followed by cooling and solidifying TCNQ the salt prior to the decomposition of the salt.
Further, it has been proposed to employ a polymer complex between polyvinyl alcohol and a 5-membered heterocyclic compound such as polypyrrole or polythiophene, as a solid electrolyte (see Japanese Unexamined Patent Publication No. 102309/1988.)
However, solid electrolytic capacitors using such TCNQ complex salt have a drawback that during the cooling and solidification after the dipping, the TCNQ complex salt crystallizes and does not adequately adhere to the dielectric oxide layer, whereby it is hardly possible to obtain the initial electrostatic capacity.
Further, the TCNQ complex salt has poor heat stability. Accordingly, when kept in a molten state at a high temperature, it undergoes thermal decomposition and changes into an electrical insulator in a short period of time, followed by quick cooling treatment, and a rather complicated apparatus for such operation will be required. Thus, there is a problem that the production cost is high.
Further, capacitors using the TCNQ complex salt have poor heat resistance against soldering (hereinafter referred to as the soldering heat resistance). Namely, by a heat shock at b 260.degree. C. for 10 seconds, the leakage current increases substantially, whereby it is extremely difficult to prepare chip type capacitors of such capacitors.
Likewise, capacitors employing a polymer complex comprising a 5-membered heterocyclic compound polymer and polyvinyl alcohol, have also poor soldering heat resistance, whereby the leakage current increases at a high temperature, and it is extremely difficult to prepare chips of such capacitors.